Friedrich August Kekulé von Stradonitz

Kekule von Stradonitz, (Friedrich) August Complete Dictionary of Scientific Biography | 2008 | Copyright Kekule von Stradonitz, (Friedrich) August

(b. Darmstandt, Germany, 7 September 1829; d. Bonn, Germany, 13 July 1896)

chemistry

Kekulé was descended from the Czech line of an old Bohemian noble family, Kekule ze Stradonič Stradonice being a village northeast of Prague. The family can be traced to the end of the fourteenth century; a branch emigrated to Germany during the Thirty Years’ War and in the eighteenth century became established in Darmstadt. Kekulé’s father, Oberkriegsrat Ludwig Carl Emil Kekule, added the accent to the family name following Napoleon’s inclusion of Hesse-Darmstadt in the Confederation of the Rhine. When Kekulé himself was ennobled by William II of Prussia, in March 1895, the terminal accent was dropped in the full style.

Kekulé attended the Gymnasium in Darmstadt, where he distinguished himself by his studiousness, aptitude for languages, and talent for drawing. His family intended him to be an architect, and he began the appropriate studies at the University of Giessen in the winter semester of 1847–1848. During the second semester, however, he so enjoyed Liebig’s chemistry course that he decided to become a chemist. Kekulé’s father had died, and the family council did not give its immediate consent to his new plan, although it was agreed that he might attend the Höhere Gewerbeschule in Darmstadt to study science and mathematics. He accordingly spent the winter to study chemistry at Giessen, beginning in the summer semester of 1849.

At Giessen, Kekulé first worked under the direction of Heinrich Will, undertaking a study on the ester of amylsulfuric acid and its salts. In the winter of 1850–1851 he began to work in Liebig’s laboratory. Liebig was at that time devoting his energies to enlarging his Chemische Briefe; he entrusted Kekulé with research on the composition of gluten and wheat bran, and cited Kekulé’s results in his twenty-seventh letter. He offered Kekulé an assistantship, but Kekulé found practical laboratory work unsympathetic to his speculative mind and decided to continue his studies abroad.

In 1851 Kekulé, upon Liebig’s advice, went to Paris, where he took courses in physics and chemistry and, in particular, became the student and friend of Charles Gerhardt. He thus came to know Gerhart’s unitary theory of chemistry, his theory of rad1icals, and his systematization of organic compounds into four types: water (H2 O), hydrogen (H2), hydrogen chloride (HCI), and ammonia (NH3); Gerhardt further made the manuscript of his Traité de chimie organique available to Kekulé. It was at this time, too, that Kekulé became interested in the problems of the philosophy of chemistry that were to concern him for some time.

Kekulé returned to Germany when his mother died. At Giessen he defended a thesis on the ester of amylsulfuric acid and was awarded the doctorate on 25 June 1852. He then became assistant to Adolf von Planta at Reichenau, Switzerland, where he remained for a year and half before taking up a similar position, on Liebig’s recommendation, with John Stenhouse at St. Bartholomew’s Hospital in London. Kekulé stayed in London from the end of 1853 until the autumn of 1855. During this time he met several other of Liebig’s former students, including A. W. Williamson, who had shortly before synthesized simple and mixed ethers that corresponded exactly to Gerhardt’s water type. (Gerhardt himself had just discovered the anhydrides of organic acids, thereby confirming the significance of this same type.)

Williamson and Kekulé became friends, and Williamson was influential in the development of Kekulé’s theoretical views. It was at Williamson’s instigation, moreover, that Kekulé began his work on the reaction of phosphorus pentasulfide on acetic acid. From this reaction Kekulé was able to isolate thioacetic acid, which he classified as a new type, hydrogen sulfide, corresponding to Gerhardt’s water and hydrogen chloride types. This work, published in 1854, marks the beginning of Kekulé’s scientific maturity. At the same time Kukulé had begun to consider, in the Gerhardt types of organic molecules, not only the radicals, but more and more the atoms themselves; he himself gave an account of a vision that he had on top of a London omnibus, in which he saw the atoms “gambolling” before his eyes. This fantasy, which was perhaps influenced by his early training in architecture, was soon to result in his theory of valence and in his structure theory.

Kekulé was, however, eager to begin a university career, and at the suggestion of Liebig and Bunsen he enrolled in the University of Hedielberg in order to the admitted there as a privatdocent. Having passed the requisite examinations, in the summer semester of 1856 Kekulé began teaching organic chemistry. He further installed, at his own expense, a lecture room and a laboratory in the first two floors of a house on the main street of Heidelberg, and it was in this private laboratory that he carried out his experiments on the chemical constitution of fulminate of mercury. Here, too, Adolf von Baeyer studied compounds of arsenic trimethyl.

During these years, too, Kekulé arrived at the concept of polyvalent radicals and introduced multiple and mixed types in a single formula of a particular compound. He introduced also the marsh gas type and worked out the theory of the tetravalence of carbon, as may be seen from an article that he published in Justus Liebigs Annalen der Chemie in 1857; in a more extensive publication of the following year he was able to state not only that the carbon atom is tetravalen in such simple compounds as CH4, CH3 Cl, CCl4, CHCl3, and CO2, but also that in compounds containing more than one carbon atom, the carbon atoms can link together in chains which can, in turn, form various polyvalent radicals. An ordered classification of organic compounds thus becomes possible. Indeed, by creating the new type CH4 and by stating the ability of carbon atoms to join up with each other, Kekulé laid the foundation of structural chemistry. He based his courses at Heidelberg on these principles, illustrating his lectures with models of individual atoms and of molecular groupings. By projecting the shadows of these models on a blackboard or on paper Kekulé obtained the “graphic formulas” that were one of his favorite teaching aids. His innovative course was a great success, and Kekulé began to consider publishing a treatise on organic chemistry.

Before he could do so, however, a chair of chemistry became vacant at the University of Ghent. The Belgian chemist Jean Servais Stas, wishing to revivify the teaching of chemistry in Belgium, strongly urged Kekulé’s nomination as full professor. Kekulé accepted the position and, at the age of twenty-nine, moved to Ghent. Stas had obtained a promise that practical chemistry would be introduced into the curriculum at Ghent and Kekulé was promised a new laboratory for both teaching and research. He was also given permission to accept private students, of whom Baeyer, one of the first, became his personal research assistant.

Despite the difficulties of adjusting to a foreign environment and of teaching in French, Kekulé soon established himself in a scholoarly mode of life. He spent the entire day in the laboratory, dedicated the evening to composing the first sections of his Lehrbuch der organischen Chemie (of which the first fascicle was printed in June 1859), and, in the hours after midnight, prepared his courses for the next day. He also found time to take the initiative in organizing the first International Congress of Chemists, which met at Karlsruhe in September 1860. The purpose of the Congress was to reduce confusion in chemical nomenclature—Kekulé was as aware as anyone of discrepancies in defining such basic concepts as the atom, the molecule, and equivalence—and to promote greater uniformity of terminology in the world chemical literature. It served a further important end as well, since it was here that Cannizzaro reestablished the importance of the Avogadro-Ampére molecular hypothesis, which had lain neglected for nearly fifty years.

Kekulé achieved some significant experimental work even before his new laboratory was ready. In particular he was concerned with the chemical structure of the organic acids and carried out, in sealed tubes, the bromination of succinic acid; from the silver salt of dibromosuccinic acid he prepared optically inactive tartaric acid; and from the silver salt of the monobromosuccinic acid he obtained maleic acid. He further demonstrated that the same family relationship exists between salicylic acid and benzoic acid as between glycolic acid and acetic acid. These researches led Kekulé to recognize the isomerism of the phenolic aromatic acids, but he was unable to account for it.

The new laboratory, constructed according to plans drawn up by Stas and Kekulé, was inaugurated in 1861, and Kekulé began to study the unsaturated dibasic acids. He was aided in this undertaking by Théodore Swarts and Eduard Linnemann, his assistants, and by one of his students, Hermann Wichelhaus. His attention had been drawn to the subject by his discovery of fumaric acid and maleic acid, two unsaturated dibasic isomers, related to succinic acid, each of which contains four carbon atoms. These acids further readily fix bromine to form two different dibromide derivatives. Having identified these entities, Kekulé was unable to interpret their structure, and the problem became more complex when he discovered three other unsaturated isomeric dibasic acids with five carbon atoms each. Since Kekulé had long held the tetravalence of carbon to be as invariable as its atomic weight, it was necessary for him to create a new theory to acknowledge the presence, in unsaturated isomers, of lacunae or double bonds between two neighboring carbon atoms. This theory of unsaturates was published in 1862; by means of it Kekulé was able to account for both the two isomers with four carbon atoms and the three acids with five carbon atoms.

The problem of unsaturated substances almost immediately came again to Kekulé’s attention in the following year, since he was writing the second part of his Lehrbuch, in which he planned to deal with the chemical structure of the aromatic compounds. The solution in this instance came to Kekulé in a vision—half awake, he saw before his eyes the animated image of a chain of carbon atoms, closing upon itself like a snake biting its own tail. He was instantly aware of the significance of such a closure, and spent the rest of the night determining the consequences of his inspired hypothesis. He arrived at a closed chain of six carbon atoms, linked alternately by three single and three double bonds and constituting the common nucleus of all the aromatic substances. He then set himself the task of experimental confirmation, but his work toward this end was delayed by various events.

At the time of his arrival in Ghent, Kekulé had met George William Drory, inspector general of the Continental Gas Association. Like Kekulé, Drory was a Protestant; they soon became close friends and Kekulé became a frequent visitor to Drory’s house. There he met and fell in love with Drory’s youngest daughter, Stéphanie, whom he married on 24 June 1862. Kekulé was thirty-two, Stéphanie nineteen. Their son, Stephan, was born the following May, and two days later Stéphanie Kekulé died. Kekulé was unable to take up his creative work for several months following her death.

He returned to his research in 1864, again taking up the search for confirmation of his benzene theory, which he had already set down in manuscript form. He first tried to do the necessary work by himself, but soon recognized the actual extent of his project and hired two assistants, Karl Glaser and Wilhelm Körner, both trained at Giessen. All the activity of the laboratory was for some time thereafter concentrated upon the trained at Giessen. All the activity of the laboratory was for some time thereafter concentrated upon the derivatives of benzene and their isomers, but Kekulé still did not publish his theory. It was only after Tollens and Fitting brought out their excellent work on the synthesis of the hydrocarbons of the benzene series that he decided to make his own work known. Thus Wurtz presented Kekulé’s benzene theory to the Société chimique de Paris on 27 January 1865, in a session presided over by Pasteur. It was subsequently published in the Bulletin de la Société chimique de Paris under the title “Sur la constitution des substances aromatiques,” and concluded with a table of formulas for benzene and similar compounds.

On 11 May 1865, Kekulé presented to the Académie associate member, a “Note sur quelques produits de substitution de la benzine,” in which he considered the geometry of the benzene nucleus and used it to determine the number of its possible monosubstituted, disubstituted, and trisubstituted isomeric derivatives. He and his associates then set out to prove these figures experimentally, and succeeded after several years’ work. They found the most diverse substituents to be those fixed on the ring or onto the lateral chains—namely the halogens and the NO2, NH2, diazo, CO2 H, SO3 H, OH and SH groups—and attempted to localize these substituents in each of the benzene isomers. On 3 August 1867, Kekulé presented to the Academy a remarkable work on this subject by Körner, “Faits pour servir à la détermination du lieu chimique dans la série aromatique.” (Körner himself stated his “absolute” method, which provided an elegant means for establishing unambiguously the ortho, meta, and para positions of the disubstituted derivatives of benzene, some seven years later.)

In addition to his work on the structure of aromatic substances, beginning in 1865, Kekulé took up the study of their azo and diazo derivatives. He began this research with a view toward incorporating the results of it in the second volume of his Lehrbuch; in addition, the subject had assumed considerable industrial importance once the potential of the intermolecular transformation of diazobenzene into aminobenzene became known. In 1866 Kekulé provided a masterful interpretation of this transformation and of the catalytic role of the aniline salts, drawing upon his new theories of the constitution of the diazo group and its mode of fixation on the benzene ring. In his wonderful researches on diazocompounds Griess prepared a new compound, called phenylendisulfuric acid, formed by interaction of concentrated sulfuric acid and diazobenzensulfate. kekulé’s interest was aroused and he proved theoretically as well as experimentally that the product was in reality a disulfonic derivative of phenol. He turned then to the study of the sulfonic derivatives of phenol and was able to clarify the double mode of action of sulfuric acid on organic matter, showing that it produces both readily decomposable sulfuric esters and highly stable sulfonic derviatives; he further emphasized the striking analogy in this respect between the sulfonyl and carbonyl groups fixed on the benzene ring..

Kekulé also discovered that sulfonic derivatives of benzene fuse with potash to create their corresponding phenols. This discovery was to become important in the industrial production of phenols. In a variation of an earlier experiment, made in London, in which he used phosphorus pentasulfide, kekulé succeeded in transforming phenol into thiophenol by substituting sulfur for the oxygen of the former. he demonstrated thereby that the oxygen of phenol is more strongly bonded to the carbon of the benzene ring than to the OH group of the fatty alcohols.

Throughout this strenuous period of research Kekulé did not neglect his teaching duties. (In 1867, for example, he published and recommended as a teaching aid a new model of the carbon atom.) But he wished to be able to teach in German again, and when he was offered the chair of chemistry at the university of Bonn—vacant since A. W. Hofmann had gone to Berlin—he accepted it gladly. He was additionally assured the directorship of a new chemical institute, the construction of which was virtually complete. In September 1867, the Blegian government accepted Kekulé’s resignation and he left Ghent for Bonn.

The new chemical institute was officially opened in 1868; the inaugural ceremonies coincided with those in celebration of the fiftieth anniversary of the university itself. Kekulé was awarded an honorary M.D. on this occasion in recognition of his contributions to theoretical chemistry. Many students were drawn to Bonn to hear his lectures and observe his class experiments and laboratory work; one of them, in 1873, was J. H. van’t Hoff, to whom kekulé’s model of the carbon atom suggested the concept of the asymmetric carbon atom of his La chimie Bans l’espace of 1875. Other students became kekulé’s direct collaborators, among them Theodor Zincke (in work on condensation of aldehydes), Hermann Wichelhaus— who had followed him from Ghent—and Thomas Edward Thorpe (on aromatic compounds), Nicolas Franchimont (on triphenylmethane and anthraquinone), Otto Strecker (on the constitution of benzene), and Richard Anschütz (on oxyderivatives of fumaric and maleic acids).

At Bonn, Kekulé found it necessary to delegate some of his teaching responsibilities to others in order to concentrate on his own research, in which he had the aid of several private assistants. his first projects were continuations of work he had begun in Ghent; he resumed his study of the sulfonic derivatives of phenol and nitrophenol, and extended his earlier investigations of camphor and oil of turpentine to include cymol, thymol, and carvacrol (the latter work was completed in 1874). More important, however, was the resumption of his attempt to provide experimental evidence for his benzene theory, particularly for the presence in the ring of three alternating souble bonds. Having observed that trimethylbenzene is formed through the condensation of three acetone molecules, Kekulé hoped to synthesize the benzene ring through the condensation of aldehyde. he was unable to obtain such a synthesis; his attempts to do so, however, resulted in an elegant series of works (published between 1869 and 1872) on the condensation of acetaldehyde. These studies treat the formation of crotonaldehyde and some of its derivative products, as well as dealing with polymerization products of aldehyde.

Kekulé also wished to demonstrate the superiority of his own formula for benzene over those put forth by A. claus, H. Wichelhaus, and A. Ladenburg. By 1872 he had created the complementary “oscillation theory,” which took into account the existence of only one bisubstituted derivative in the ortho position, rather than two. He thus permitted the delocalization of single and double bonds, which he had considered to be fixed in his earlier theory.

In the same year Kekulé and Franchimont succeeded in synthesizing triphenylmethane, the fundamental hydrocarbon in rosaniline dyes, and also obtained anthraquinone in the course of preparing benzophenone. The elucidation of the structure of these compounds proved crucial to the development of synthetic dyes; the subsequent rapid growth of the German aniline dye industry, based on the triphenylmethane group and authraquinone, was its direct result.

The growing number of Kekulé’s students and co-workersc soon necessitated an expansion of the chemical institute, to which a number of new workrooms were added in 1874 and 1875. During this period Kekulé was offered the chair of chemistry at the University of Munich, which had become vacant with the death of Liebig; he declined the post, however, and recommended Adolph von Baeyer in his stead. At the same time, his health had begun to fail. Twenty years of overexertion had begun to take their toll, as had an unfortunate second marriage to his former housekeeper, a woman much younger than he, who was incapable of relieving him of his cares. A month after this marriage, too, Kekulé contracted measles from his son and suffered prolonged aftereffects. he nonetheless continued to serve the university, being elected rector in 1877, on which occasion he gave an address on the scientific goals and accomplishments of chemistry. upon completion of his office the following year, he spoke upon the principles of higher education and educational reform.

At about the same time Kekulé resumed work on the Lehrbuch, in collaboration with Gustav Schultz, Richard Anschütz, and, slightly later, Wilhelm la Coste; but the rapid growth of chemistry at that time did not allow them to maintain the original plan of composition, and the work was never completed. Although volume III appeared in 1882, volume IV, published in 1887, consisted of only one of the planned sections. From 1879 to 1885 Kekulé also engaged in research, primarily experiments designed to support his own benzene theory against the prismatic formula advocated by August Ladenburg. Ludwig Barth had become a partisan of the latter theory, arguing from the formation of carboxytartronic acid from pyrocatechol. In 1883 Kekulé was able to show that this acid is simply tetraoxysuccinic acid, the formation of which from pyrocatechol was better explanined by his own hexagonal theory. In a series of investigations on trichlorophenomalic acid made with Otto Strecker in the following year, Kekulé again corroborated his own thesis and confirmed the superiority of his own formula, which provided an atom-by-atom explanation of the formation of β-trichloracetylarcrylic acid through the oxydochlorination of quinone.

A high point in Kekulé’s career occurred in 1890, when he read his paper “Ueber die Konstitutionen des Pyridins” to the general assembly of the Deutsche Chemische Gesellschaft in Berlin on 10 March. The communication summed up the investigations on pyridine, of which the formula is comparable to that of benzene, that he had carried out since 1886. The day after this presentation, a great celebration was held to honor Kekulé on the occasion of the twenty-fifth anniversary of his benzene theory. kekulé in thanks, gave a remarkable speech in which he reviewed his life’s work and made public for the first time the details of his visionary solution of the benzene ring.

Although he had grown deaf by this time, Kekulé continued to teach and carry out administrative duties. In 1892 he also prepared formic aldehyde in the pure state, thus extending his earlier work on the condensation of the aldehydes. His health was again seriously impaired following an attack of influenza, and he died shortly thereafter. He was buried in the family vault in the cemetery of Poppelsdorf; a bronze statue of him, paid for largely by subscription from the German dyestuffs industry, was erected, facing his chemical institute, in 1903. BIBLIOGRAPHY

Kekulé’s works were collected, with a biography, by Richard Anschütz to honor him at the centenary of his birth: August Kekulé, I, leben und Wirken, II, Abhandlungen, Berichte, kritiken, Artikel, Reden (Berlin, 1929).

Secondary literature includes G. V. Bykov, August Kekulé (Moscow, 1964), in Russian; j. Gillis, “Auguste Kekulé et son oeuvre, réalisée à Gand de 1858 à 1867,” in Mémoires de I’ Académie royale de Belgique. Classe des sciences,37 (1966), 1–40; Francis R. Japp, “Kekulé Memorial Lecture,” in Journal of the Chemical Society, 73 (1898), 97–138; and R. Wizinger-Aust et al., Kekulé und seine Benzolformel. . . (Weinheim, 1966).

Jean Gillis

cf.: http://de.wikipedia.org/wiki/August_Kekulé